1. Field of the Invention
The present invention relates generally to coating compositions, and more particularly to polyorganosiloxane compositions forming mar resistant coatings on substrates.
2. Prior Art
A variety of substrates, including those made of glass, plastic, or metal and concrete, are usefully coated with protective films. It is generally desirable that protective coatings have good weathering, adhesion and resistance to thermal and mechanical shock, heat, humidity and common chemicals, and be practical to apply, dry and cure. These properties are more difficult to achieve on plastic substrates than many other substrates.
Some plastics are desirable substitutes for glass due to lighter weight, economically advantageous fabrications, and breakage resistance; but, commercially available plastic surfaces are less abrasion, mar, and scratch resistant than glass. Thus, protective coatings for plastic substrates are of particular interest.
Much effort has been done in this field, and several different technical approaches have been described. A majority of the work has been with polyorganosiloxanes crosslinked solely by condensation of silanol groups. This approach is typified in U.S. Pat. Nos. 3,986,997 and 4,027,073, assigned to Dow Corning. A further major area of development has been with fluoropolymer coatings.
Also, epoxy silanes cured with Lewis acids have been the subject of much research and development. Perhaps one of the more commercially successful approaches has been the use of epoxy silanes cured with metal esters, particularly titanates, exemplified by U.S. Pat. No. 4,042,769.
Among various types of coatings or primers known to the art are several epoxy and amino based polyorganosiloxanes. Six patents generally disclose an epoxyalkylalkoxysilane and/or an aminoalkylalkoxysilane for use in particular coating formulations. Thus, for example, U.S. Pat. No. 3,166,527, to H. H. Ender, issued Jan. 19, 1965, discloses finishing compositions for treating metal surfaces. U.S. Pat. No. 3,837,876, to T. Mayuzumi and Y. Inoue, issued Sept. 24, 1974, discloses polyorganosiloxane compositions useful in blends for sealant or primer coatings. U.S. Pat. No. 3,961,977, to H. Koda et al., issued June 8, 1976 discloses a coating composition for application to molded articles. U.S. Pat. No. 4,196,014, to S. Taniyama et al., issued Apr. 1, 1980, discloses a surface-treating paint for shaped plastic articles, and U.S. Pat. No. 4,241,116, to S. Taniyama et al., is an improvement on U.S. Pat. No. 4,196,014. Of these six, U.S. Pat. Nos. 3,961,977, 4,196,014, and 4,241,116 disclose that such silanes can be hydrolyzed with sufficient water to hydrolyze 10-40% of the available alkoxy groups.
However, the coatings or primers known to the art based upon epoxyalkylalkoxysilane and aminoalkylalkoxysilane have had several disadvantages. For example, stability of the coating solutions disclosed in U.S. Pat. No. 3,961,977 is achieved by high dilution, which is undesirable in that necessary (desirable) film thickness can only be achieved by the application of several coats. Also, compositions as taught by this patent are believed to be inferior in abrasion resistance and have been criticized in the literature as having poor resistance to weathering, to develop cracks, and/or to peel and humidity.
The present invention is directed to overcoming one or more of these problems.